Hi scatcat,
When I first joined Axiom and did a bunch of listening tests to all of the speakers, out of curiosity I put a single VP150 vertically on a stand and, like you, I was astonished at how good it sounded. I even mentioned this to Ian at Axiom (I think I said something like, "You know, they sound so good we could sell pairs of those for special applications. . .")
Of course, there might be imaging issues and I have not explored it further, but next time I'm at the plant, it might be interesting to do a blind comparison of a pair of VP150s used vertically with a pair of M60s!
Regards,
